In a distributed storage system, erasure coding that makes data redundant by using parity for restoring erased data is used. In the distributed storage system using the erasure coding, when a server stores data in a storage device, parity is generated and the data and parity are distributed and stored in a plurality of storage devices. In the following explanation, a storage device storing data is also referred to as a data storage device. Further, a storage device storing parity is also referred to as a parity storage device.
Then, the server keeps the parity storage device in the power-off state except for during the period of time during which parity is read in order to restore data, and during the period of time during which new parity is written. Due to this, the server reduces the period of time during which the parity storage device is in the power-on state and thus reduces power consumption. In the following explanation, the power-off state is also referred to simply as an off state and the power-on state is also referred to simply as an on state.
Further, when storing data in the data storage device in the case where the parity storage device is in the off state, the server generates new parity and writes the parity in an empty area of the data storage device. Then, when reading parity for data restoration, the server reads the parity stored in the data storage device. Due to this, the server reduces the frequency of reading and writing of parity from and to the parity storage device and thus further reduces the power consumption. When there is no longer an empty area in the data storage device, the server performs processing to bring the parity storage device into the on state and to write the parity stored in the data storage device back to the parity storage device. In the following explanation, as described previously, in the case where the storage device that is the storage destination of data is in the off state, data is stored temporarily in another storage device. Processing to write the data stored in another storage device back to the storage device that is the storage destination of the data when the storage device that is the storage destination of the data enters the on state is also referred to as write off-loading.
As a related technique, a technique as follows is known. The controller determines whether or not the empty capacity of the nonvolatile memory is smaller than a predetermined data size upon receipt of a command from the host device to write data to be written that is stored in the buffer to the hard storage device, and then, in the case where the empty capacity is smaller than the predetermined data size, in order to increase the empty capacity, the controller deletes synchronized data, which is data of the predetermined data stored in the nonvolatile memory, that is identical to that existing on the hard storage device, and the controller writes the data to be written that is stored in the buffer to the nonvolatile memory.
Further, as a related technique, a technique as follows is known. That is, the file device stages a file segment F1-2 of a file F1 on a cache CS and registers the file segment F1-2 to an order lower than that of a file segment F1-1 in an LRU table, and then the file device stages a file segment F2-2 of a file F2 on the cache CS from a storage device DS and registers the file segment F2-2 to an order lower than the file segment F2-1 in the LRU table, and in this manner, the file device registers a file segment closer to the front of the file to a higher order, and thereby the file device makes the probability higher that a file segment closer to the front of the file exists on the cache CS.
Furthermore, a technique as follows is also known. That is, in a storage system, in the case where a predetermined storage device will not be made use of because the power is in the off state or the power is an overloaded state when a request to store data in the predetermined device is received, the data is stored in another storage device, and when the predetermined storage device becomes available again, the data that was stored in the other storage device is acquired and written to the predetermined storage device (e.g., Japanese Laid-open Patent Publication No. 2009-18134, Japanese Laid-open Patent Publication No. 06-119218, and U.S. Pat. No. 8,074,014).
In the distributed storage system that uses the storage device control technique described previously, all the data storage devices are kept in the on state so that the parity storage device is not activated at the time of reading, and therefore, there is a problem wherein it is not possible to reduce the power consumption of the data storage device.